S. final [mf], [nθ], [ŋx], [lθ] became [mp], [nt], [ŋk], [lt]; [-{mf|nθ|ŋx|lθ}] > [-{mp|nt|ŋk|lt}]

S. final [mf], [nθ], [ŋx], [lθ] became [mp], [nt], [ŋk], [lt]; [-{mf|nθ|ŋx|lθ}] > [-{mp|nt|ŋk|lt}]

In Sindarin and Noldorin, where combinations of nasals and voiceless spirants (mph, nth, nch) became final after short final vowels vanished, the spirant became a stop again (mp, nt, nc). The same is true of the combination lth (becoming lt) but not rth. Tolkien described this change in a couple places:

Similarly in oldest Sindarin, mp, nt, nc were preserved. In North Sindarin they universally became mf, nth, ŋch at the same time as lt, lp, lc and rt &c. > lth, lf, lch. The normal development in North Sindarin was then restopping when final mp, nc, lt but nth, lf, lch, rth, rf, rch remained spirantal and medial nch > ¯ch. Later again nth > nt finally ... In Oss[riandic], West & South Sindarin changes proceeded as far as mph, nth, ŋch. Then lt, mp, np, nc restopped finally (phonetic notes from 1962, PE17/131).
In the Northern dialect (which perished in the course of the war against Morgoth) nt, nc, mp remained unchanged. In the Southern dialects nt, ñk, mp remained when standing finally — or more probably the spirant was re-stopped in this position, for similarly final lth > lt, though rth remained finally (The Rivers and Beacon-hills of Gondor, late 1960s, VT42/27).

Final mp, nt, nc, lt are common in both Sindarin and Noldorin, but finding explicit examples of the restopping is difficult, since Tolkien rarely showed the intermediate changes. There is, however, one example from a deleted entry in The Etymologies:

Conceptual Developments: Final mp, nt, nc, lt are also widespread in the Gnomish of the 1910s and Early Noldorin of the 1920s. Meanwhile, mf, nth, nch, lth occur medially but not finally, so it is likely this sound change dates back to Tolkien’s earliest ideas of the language, as noted by Roman Rausch in his Historical Phonology of Goldogrin (HPG/§2.2).

References ✧ PE17/131; VT42/27

Order (03800)

After 02300 short final vowels vanished ON. maltha > N. malt EtyAC/SMAL
Before 03900 nasals vanished before spirantal clusters ON. inpanta > in-fant > N. ifant Ety/YEN

Phonetic Rule Elements

[-mf] > [-mp] ✧ VT42/27 (mf > mp; finally)
[-nθ] > [-nt] ✧ VT42/27 (nth > nt; finally)
[-ŋx] > [-ŋk] ✧ VT42/27 (nch > ñk; finally)
[-lθ] > [-lt] ✧ VT42/27 (lth > lt; finally)

Phonetic Rule Examples

gamf > gamp -mf > -mp gampa > S. gamp ✧ VT47/20
nimf > nimp -mf > -mp NIK > S. nimp ✧ PE17/168
nimf > nimp -mf > -mp nimpĭ > S. nimp ✧ VT48/18
exxanθ > exxant -nθ > -nt et-kantē > S. echant ✧ PE17/42
sanθ > sant -nθ > -nt SAT > S. sant ✧ VT42/20
fliŋx > fliŋk -ŋx > -ŋk AT. phlinke > ᴸN. flinc ✧ PE21/72
fliŋx > fliŋk -ŋx > -ŋk philinkĭ > ᴸN. flinc ✧ PE21/81
laŋx > laŋk -ŋx > -ŋk (G)LAN > S. lanc ✧ VT42/8
r̥aŋx > r̥aŋk -ŋx > -ŋk SRAK > S. rhanc ✧ PE17/154
truŋx > truŋk -ŋx > -ŋk turunko > S. trunc ✧ PE21/80

N. final [mf], [nθ], [ŋx], [lθ] became [mp], [nt], [ŋk], [lt]; [-{mf|nθ|ŋx|lθ}] > [-{mp|nt|ŋk|lt}]

GS/§4.117 @@@

Order (03600)

After 02100 short final vowels vanished ON. maltha > N. malt EtyAC/SMAL
Before 03700 nasals vanished before spirantal clusters ON. inpanta > in-fant > N. ifant Ety/YEN

Related

Phonetic Rule Elements

[-mf] > [-mp]
[-nθ] > [-nt]
[-ŋx] > [-ŋk]
[-lθ] > [-lt]

Phonetic Rule Examples

dolθ > dolt -lθ > -lt ᴹ√NDOL > N. dolt ✧ Ety/NDOL
dylθ > dylt -lθ > -lt ᴹ√NDOL > N. dylt ✧ Ety/NDOL
m̥alθ > m̥alt -lθ > -lt ON. maltha > N. malt ✧ EtyAC/SMAL
talθ > talt -lθ > -lt ᴹ√TALÁT > N. talt ✧ Ety/TALÁT
gamf > gamp -mf > -mp ᴹ√GAP > N. gamp ✧ Ety/GAP
l̥imf > l̥imp -mf > -mp ᴹ√LINKWI > N. lhimp ✧ Ety/LINKWI
nimf > nimp -mf > -mp ᴹ√NIK-W > N. nimp ✧ Ety/NIK-W
tumf > tump -mf > -mp ᴹ√TUMPU > N. tump ✧ Ety/TUMPU
xamf > xamp -mf > -mp ᴹ√KHAP > N. hamp ✧ Ety/KHAP
anθ > ant -nθ > -nt ᴹ√ANA¹ > N. ant ✧ Ety/ANA¹
atlanθ > atlant -nθ > -nt ᴹ√TALÁT > N. atlant ✧ Ety/TALÁT
danθ > dant -nθ > -nt ᴹ√DANT > N. dant- ✧ Ety/DAT
erianθ > eriant -nθ > -nt ᴹ√ANA¹ > N. Eriant ✧ Ety/AR¹
infanθ > infant -nθ > -nt ON. inpanta > in-fant > N. ifant ✧ Ety/YEN
janθ > jant -nθ > -nt ᴹ✶yantā > N. iant ✧ Ety/YAT
kanθ > kant -nθ > -nt ᴹ√KAT > N. cant ✧ Ety/KAT
l̥anθ > l̥ant -nθ > -nt ᴹ√LAT > N. lhant ✧ Ety/LAT
l̥unθ > l̥unt -nθ > -nt ᴹ√LUT > N. lhunt ✧ Ety/LUT
menθ > ment -nθ > -nt ᴹ√MET > N. ment ✧ Ety/MET
minθ > mint -nθ > -nt ᴹ√MIT > N. mint ✧ EtyAC/MIT
panθ > pant -nθ > -nt ON. panta > N. pant ✧ Ety/KWAT
penθ > pent -nθ > -nt ᴹ✶kwentā > N. pent ✧ Ety/KWET
ranθ > rant -nθ > -nt ᴹ√RAT > N. rant ✧ Ety/RAT
taŋxanθ > taŋxant -nθ > -nt ON. tankánte > N. tangant ✧ PE17/44
tinθ > tint -nθ > -nt ᴹ√TIN > N. tint ✧ Ety/TIN
θenθ > θent -nθ > -nt ON. sthinta > N. thent ✧ Ety/STINTĀ
fliŋx > fliŋk -ŋx > -ŋk ᴹ✶philínkē > N. flinc ✧ PE21/56
iŋx > iŋk -ŋx > -ŋk ᴹ√INK/INIK? > N. inc ✧ Ety/INK
l̥aŋx > l̥aŋk -ŋx > -ŋk ᴹ√LANK > N. lhanc ✧ Ety/LANK
l̥aŋx > l̥aŋk -ŋx > -ŋk ᴹ√LAK¹ > N. lhanc ✧ EtyAC/LAK¹
r̥aŋx > r̥aŋk -ŋx > -ŋk ON. ranko > N. rhanc ✧ Ety/RAK
r̥iŋx > r̥iŋk -ŋx > -ŋk ᴹ✶rinki > N. rhinc ✧ Ety/RIK(H)
taŋx > taŋk -ŋx > -ŋk ᴹ√TAK > N. tanc ✧ Ety/TAK
tiŋx > tiŋk -ŋx > -ŋk ᴹ✶TINKŌ > N. tinc ✧ Ety/TINKŌ
θaŋx > θaŋk -ŋx > -ŋk ON. sthanka > N. thanc ✧ Ety/STAK

G. final voiceless spirants restopped after consonants; [-C{fθx}] > [-C{ptk}]

HSG/§2.2, 2.4

Order (04300)

After 03900 [x], [ɣ] sometimes became [i]
After 04100 [f], [v] sometimes became [u]
Before 06300 [ð] became [θ] finally or before a consonant ᴱ√qṛđ > G. cwardh GL/28

Phonetic Rule Elements

[-Cf] > [-Cp]
[-Cθ] > [-Ct]
[-Cx] > [-Ck]
[-Cf] > [-Cf]
[-Cθ] > [-Cθ]
[-Cx] > [-Cx]

Phonetic Rule Examples

falx > falc -Cx > -Ck ᴱ√FḶKḶ > G. Falc ✧ LT2A/Glorfalc
gʷiŋx > gʷiŋk -Cx > -Ck ᴱ√GWINI > G. gwinc ✧ LT1A/Morwinyon
helx > helk -Cx > -Ck ᴱ√χele-k > G. helc ✧ GL/48
ndolx > ndolk -Cx > -Ck ᴱ√ndolo- > G. dolc ✧ GL/30
arx > arx -Cx > -Cx ᴱ✶ṝkă > G. arch ✧ PE13/110
amrinθ > amrint -Cθ > -Ct ᴱ✶ambarinþǝ > G. amrint ✧ PE13/110
balθ > balt -Cθ > -Ct ᴱ√vḷt > G. balt ✧ GL/23
benθ > bent -Cθ > -Ct ᴱ√VENE > G. bent ✧ LT1A/Glorvent
blanθ > blant -Cθ > -Ct ᴱ√PALA > G. blant ✧ LT1A/Palúrien
falθ > falt -Cθ > -Ct ᴱ√fal-² > G. falt ✧ GL/33
ganθ > gant -Cθ > -Ct ᴱ√yṇt > G. gant ✧ GL/37
gilθ > gilt -Cθ > -Ct ᴱ√Gil- > G. gilt ✧ GL/38
gʷalθ > gʷalt -Cθ > -Ct ᴱ√gwal > G. gwalt ✧ GL/44
gʷanθ > gʷant -Cθ > -Ct ᴱ√VANA > G. gwant ✧ LT1A/Vána
gʷenθ > gʷent -Cθ > -Ct ᴱ√gwene- > G. gwent ✧ GL/45
gʷenθ > gʷent -Cθ > -Ct ᴱ√gwed² > G. gwent ✧ GL/46
gʷerθ > gʷert -Cθ > -Ct ᴱ√gwer- > G. gwert ✧ GL/46
kʷenθ > kʷent -Cθ > -Ct ᴱ√QETE > G. cwent ✧ LT2A/Tôn a Gwedrin
lenθ > lent -Cθ > -Ct ᴱ√LENE > G. glent ✧ LT2A/Glend